Back in the early days when things weren't so frenetic, the Fantastic Four would have time to relax and unwind a bit between cosmic threats and internal strife in the superhero community. Mister Fantastic would tinker around with complicated machinery to refract tides, the Thing would read newspapers and smoke cigars while weightlifting double decker busses, Johnny would be throwing darts made of fire at a dartboard with a picture of Spider-Man on it, and Sue? Sue would be in the Wig Room.
Because a girl is a girl, even when she's a full-fledged partner of the Fantastic Four. And don't all girls while away the hours trying on different coloured wigs?